When photolithography is used to manufacture a semiconductor device, and the like, a projection exposure apparatus has, so far, been used to expose a pattern of a reticle onto a plate via a projection optical system. A growing demand for a finer semiconductor device promotes a reduction to practice of a projection exposure apparatus that uses EUV light having a wavelength (such as 5 nm to 20 nm) shorter than that of ultraviolet light.
For precise transfer of a mask pattern onto a plate with a desired magnification, a projection optical system requires a high imaging performance that maintains aberration to be as low as possible. Particularly, a growing demand for a finer semiconductor device causes the transfer performance to be sensitive to an aberration of the projection optical system. Therefore, a requirement is to measure a wavefront aberration of the projection optical system with high precision.
A lateral shearing interferometer (“LSI”) is one known apparatus that highly, precisely, measures the wavefront aberration of a projection optical system for EUV light without extremely accurate alignment of a pinhole, unlike a point diffraction interferometer (“PDI”). In general, the LSI arranges a pinhole mask that has one pinhole on an object plate of the tested optical system. The image of the pinhole is projected on an image plane, while being influenced by aberration of the projection optical system. A diffraction grating is located between the image plane and the tested optical system, and the light beams transmitted through the tested optical system slightly laterally shift according to their diffraction order. As a result, the transmitted light beams slightly laterally shift on an observation plane subsequent to the image plane, and overlap and interfere with each other, generating an interferogram (interference pattern). The interferogram gives a laterally differential wavefront of the original wavefront, which is reconstructed to the original wavefront through integration.
To extract from the pinhole a light beam with enough intensity to measure a wavefront, it is necessary to use a bright light source and to condense the light from this light source to the pinhole. One conceivable bright light source is an undulator light source that is inserted into an electron storage ring, but this requires large-scale facilities, incurring a high cost. A small light source, particularly one which also serves as an exposure light source, is preferable for wavefront measurements used in an assembly process or at an installation place of an exposure apparatus.
On the other hand, a light beam from an exposure light source, such as laser produced plasma (“LPP”) light and discharged produced plasma (“DPP”) light, has such a low directivity that it is very difficult to condense the light to the pinhole. When this exposure light source is used for wavefront measurements, an amount of the light that passes the pinhole is too small to produce a sufficiently intense interference image on the observation plane for wavefront measurements.
One proposed scheme arranges many reflection dots on the object plane of the tested optical system to improve the light use efficiency. See, for example, Japanese Patent Application, Publication No. 2004-219423. This reference also discusses arranging a diffraction grating on an image plane position of the tested optical system. The diffraction grating laterally shifts diffracted light beams with different orders and causes interference. A phase shift method finds a differential wavefront between the laterally shifted diffracted light beams, and calculates the wavefront of the tested optical system.
In general, a lateral shearing interferometer (LSI) using the phase shift method needs to take multiple interference images while shifting, by a specific quantity, a phase difference between diffracted light beams with respective orders, and to similarly take multiple interference images while changing the above lateral shifting direction. Accordingly, this type of LSI should take two pairs of interference images, requiring a long period of time for measurements. A problem of measurement error occurs unless an optical component, such as a diffraction grating, is maintained extremely stable in its height direction when moved, while these interference images are being taken.